During drilling or after the completion of a well, it is often desirable to obtain information about the formation surrounding the wellbore. In wells completed into or through zones in the formation containing hydrocarbons, it is desirable to have techniques of obtaining information about these producing zones. Well logs are often run to determine such parameters as resistivity, conductivity and other parameters from which the properties of the oil and the producing formation can be deduced to give a clearer picture of the environment surrounding the wellbore.
One measurement technique involves the use of geophones which are lowered into the wellbore while surface seismic sources generate waves to pass through the geologic formation. The geophones sense these waves. Subsequent processing of the recordings derived from these waves provide a clearer picture of the environment surrounding the wellbore. However, the weathering layer of the earth's crust attenuates a great deal of the energy from the seismic sources before it reaches the zones of interest in the formation surrounding the wellbore. Providing a seismic source within a wellbore below the weathering layer and spaced from a different wellbore in which the geophones are located would remove the effects of the weathering layer.
Typically, a well is completed into a formation by cementing a liner within the wellbore. Any source used within the wellbore must be capable of imparting the desired amount of energy into the formation to be detected either at the surface or in an adjacent wellbore. However, the form of the energy must not input shear or compressive borehole stresses capable of separating the liner from the cement or casing. These stresses should be less than the maximum recommended shear stress on the casing cement interface, which is about 20 psi, as specified by the American Petroleum Institute (API RP2A Oct. 22, 1984).
FIG. 12 and FIG. 13 are graphs comparing the borehole stresses and effective energies between known destructive downhole sources, namely, a 1.1 lb (500 gm) dynamite charge, a 40 cu. inch (655 cc) airgun, and a 4,000 lbf output (18,000 newtons) seismic source of the invention. The values are applicable around 100 hz, a frequency which is within the usual frequency band. The figures show the seismic vibratory source achieves high effective energies with low borehole stresses. The 10 psi borehole stress induced by the seismic source of the invention is only half of the recommended maximum induced shear stress on the casing cement interface according to American Petroleum Institute standards (API RP2A Oct. 22, 1984).
Currently, a technique called "Vertical Seismic Profiling", (VSP), is used to obtain information about particular zones of interest within the formation surrounding the wellbore. Typically, this technique requires lowering a geophone into a wellbore while providing various surface seismic energy sources, such as dynamite, to impart seismic energy into the formation to be received by the geophone. The geophone is lowered to a specific depth within the wellbore and a surface seismic energy source is located on the surface to impart the seismic energy into the formation which is detected by the geophone and recorded. The geophone is moved to a different depth and the process is repeated. The surface seismic energy source is moved to various locations at various offsets from the wellbore and the process is repeated. This process is extremely costly and time-consuming. The repeated use of destructive surface sources renders this process unsuitable for use in populated areas and unpopular with many surface landowners.
A great deal of the time and expense could be eliminated if a nondestructive seismic source could be placed in a wellbore to be used in a reverse VSP (RVSP) process. The RVSP process requires a source in a wellbore and numerous geophones on the surface. The seismic energy generated from the seismic source is detected by the geophones on the surface. Since numerous geophones can be laid out in a two-dimensional array from the wellbore, the location of the geophones can remain constant while the seismic source moves instead of requiring movement both of the seismic source and the geophones. Assuming a nondestructive seismic source can be located within a wellbore spaced from an adjacent wellbore containing one or more geophones, superior crosswell tomography processes of production wells can be performed because the source can be located below the attenuating weathering layer of the earth's crust. In studying the subterranean formations, it would be desirable to receive and analyze a host of different types of seismic waves, e.g., P-waves, S.sub.V -waves, and S.sub.H -waves. Thus, it would be highly desirable to have an apparatus capable of nondestructively generating one or more of these types of waves within a wellbore for reception by geophones located either at the surface or in an adjacent wellbore to deduce information about the formation. It would also be desirable to have a source which can nondestructively generate frequencies in excess of 100 Hz below the weathering layer to perform crosswell tomography and crosswell profiling. Crosswell tomography between adjacent wellbores cannot be adequately performed at the present time using oil wells because present available downhole seismic sources, e.g., dynamite or the air gun are all impulsive forces generating shear stress far in excess of API standards for detectable surface seismic waves. Attempts to tomographically image reservoirs using surface data gathering sources, e.g., geophones, have failed due to the attenuation and filtering of the higher frequencies by the weathering layer, as well as unfavorable source-receiver configuration.